This invention relates in general to vertical deflection drive circuitry in the cathode ray tube of a video display and in particular to a push-pull type yoke current deflection system with an S-correction feedback control loop for generating a more accurate sawtooth deflection waveform.
An image is displayed on the face of a cathode ray tube (CRT) is repeatedly scanning an electron beam across the face of the CRT both in horizontal and vertical directions. The sweep is caused by the application of sawtooth current waveforms to deflection windings in the CRT. If a linear sawtooth waveform is applied to the deflection winding, display distortions appear on the face of the CRT. These distortions are caused by the electron beam striking a flat surface such that equal increments of beam deflection do not intersect equal areas on the face of the CRT. This phenomena is referred to as the flat face effect and is corrected for by means of the so-called S-correction factor which operates to compress the beginning of the sawtooth current waveform and the end of the sawtooth waveform to give the appearance of an S. Various attempts to eliminate the aforementioned sources of video presentation distortion are documented in the prior. See U.S. Pat. No. 3,842,311 and 3,991,345.
Another source of nonlinearity in electron beam deflection is the deflection coil itself. Because of the high inductance and relatively low resistance characteristic of electromagnetic deflection coils, the voltage applied across the coil is different than the current transmitted through the coil. This situation creates nonlinear current fluctuations in the deflection coil due to these large inductances. Finally, amplifier instabilities, or instabilities in transistor gain, in the deflection circuit cause nonlinearities in the sawtooth current waveform used to drive the deflection coil.
The present invention eliminates the nonlinear effects of the aforementioned drive circuit characteristics. The result is a DC-coupled, direct drive vertical deflection system in which an input sawtooth current waveform is directly coupled to the vertical deflection yoke. The present invention represents an inexpensive, efficient and energy saving approach to electron beam deflection in a CRT.